Method and apparatus for converting nitrogen dioxide to nitric oxide

ABSTRACT

A copper catalyst maintained in the temperature range of 220° to 240°C. is employed to convert nitrogen dioxide to nitric oxide, as in the measurement of oxides of nitrogen by chemiluminescent techniques. The converter comprises a body having a bore receiving a copper tube coaxially, the tube being spaced from the wall of the bore so that a gas containing nitrogen dioxide may flow along the exterior of the tube and then through the interior of the tube, which contains copper granules held in place by plugs of copper turnings. The temperature of the body is maintained by heater tape wound about the body, and the body is enclosed within an insulated container.

BACKGROUND OF THE INVENTION

This invention is concerned with the conversion of nitrogen dioxide tonitric oxide, to permit the measurement of oxides of nitrogen bychemiluminescent techniques, for example.

Instruments which measure the concentration of oxides of nitrogen bychemiluminescent techniques are well known. In general, such instrumentsdetect the light produced by the reaction of ozone and nitric oxidewhich are mixed in the detector head, the light resulting from thereaction being sensed by a photomultiplier tube. Since thechemiluminescent reaction requires nitric oxide to work, if aconcentration of nitrogen dioxide is to be measured, it must first beconverted to nitric oxide. Hence, various types of converters haveheretofore been proposed for use in instruments for determining oxidesof nitrogen in ambient air or emission sources and also for use withcalibration units having a nitrogen dioxide source.

Converters employing various types of catalytic materials are wellknown. However, the converters employed heretofore have suffered fromone or more of the following disadvantages or deficiencies: high cost,restricted range in terms of the concentrations of NO₂ which can beconverted, short lifetime, high operation temperatures, production ofinterfering compounds, inefficiency, complexity, and inability tooperate in the presence of other gases or under certain ambientconditions.

BRIEF DESCRIPTION OF THE INVENTION

It is accordingly a principal object of the present invention to providean improved method and apparatus for converting nitrogen dioxide tonitric oxide, a method and apparatus which overcome, avoid, or mitigatethe disadvantages of the prior art.

Briefly stated, a preferred embodiment of the present invention employsa copper catalyst maintained in the temperature range of about 220° to240°C. The catalyst comprises a copper tube containing copper granulesheld in the tube by plugs of copper turnings. The tube is received in abore of a body and is spaced from the wall of the bore so that the gasto be treated may be passed along the exterior of the tube within thebore and then through the interior of the tube, the gas being heated asit flows within the body, and the body being heated by an electricheater. The body is supported in an insulated housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in conjunction with theaccompanying drawings, which illustrate a preferred and exemplaryembodiment, and wherein:

FIG. 1 is contracted elevation view of the invention with the housingand body partially broken away to reveal internal parts;

FIG. 2 is a contracted elevation view, partly in section, illustratingthe copper tube and associated fittings and exhaust conduit;

FiG. 3 is a longitudinal sectional view of the body; and

FIG. 4 is a block diagram illustrating the utilization of the converterof the invention in a chemiluminescent detection system.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the converter apparatus 10 of the invention maycomprise a body 12 supported within a housing 14. As shown in FIG. 3,the body 12 may comprise a cylindrical block of stainless steel providedwith an elongated central bore 16. One end of the bore terminates shortof the corresponding end 18 of the block and the other end communicateswith an enlarged internally threaded bore 20 constituting an extensionof the bore 16. A threaded side bore 22 communicates with bore 16adjacent to bore 20.

Received within bore 16 is a tube 24, preferably of copper. Tube 24 ispreferably silver soldered within a bore of a threaded fitting 26, whichis threaded into the bore 20 of the body or block 12. An exhaust conduit28 communicates with tube 24 via a further fitting 30 threaded intofitting 26. Fittings 26 and 30 may be of the Swagelok type. As shown inFIG. 1, the body 12 may be suspended from the cover 32 of the housing 14(also cylindrical) the exhaust conduit 28 extending through a hole inthe cover, then being bent at right angles along the cover and beingsecured thereto by a clamp 34. In the form shown, the cover disc 32 isretained upon the cylindrical side wall of the housing by means offasteners 36, the opposite end of the housing being permanently closed.A pipe 38 for the supply of gas to be treated is coupled to the body viaelbow 40 threaded into bore 22 of the body and provided with a fitting42, which may be of the Swagelok type, for gas tight connection to pipe38, which passes through a hole in cover 32.

As shown in FIG. 1, the external diameter of tube 24 is less than theinternal diameter of bore 16, so that the tube is spaced from the wallof the bore. Also, the end of tube 24 adjacent to the end 18 of the bodyterminates short of the corresponding end of the bore to providecommunication between the bore and the interior of the tube 24. It isthus apparent that gas supplied by pipe 38 may enter the space betweenthe bore wall and the exterior wall of tube 24, travel along theexterior of tube 24 in one direction and then enter the interior of thetube and travel along the tube in the opposite direction, beingexhausted from the tube via conduit 28. Fitting 26 prevents movement ofthe gas through bore 20 of the block until the gas has followed the pathjust described.

The interior of tube 24 is filled with copper granules 44 held in thetube by plugs 46 of copper turnings or shavings, such as the typeproduced by the machining of copper stock. The copper granules may belight copper chips sold by Fisher Scientific Company. The portion ofbore 16 adjacent to end 18 of the block may also be filled with copperturnings as indicated at 48, to aid in retention of the particulate bedin tube 24. Plug 46 at the top of tube 24 is not shown.

To supply heat to the block an electric heater 50 is wrapped about theexterior of the block. Preferably a preliminary wrapping of glass tapeis provided, then the heater, in the form of a heating tape, then anasbestos insulating tape, and finally an aluminum tape, all wrapped insequence about the block. Prior to the foregoing wrapping, a strainrelief may be provided at 52, secured to the block by means of a screw.Heater lead wires 54 are then connected to the heater via "wire-form"contacts held by the strain relief. A thermistor (not shown) taped tothe body 12, and connected to lead wires via the strain relief also, isemployed in a conventional temperature control circuit for controllingthe electric energy supplied to the heater 50, thereby to control andmaintain the temperature of the block. A thermocouple (not shown) may besimilarly mounted and connected for temperature measurement. Theinterior of housing 14 is filled with insulation 54, such as glass wool,to isolate the block from ambient temperature.

In operation, gas to be treated, containing nitrogen dioxide, issupplied to the block by pipe 38, the gas passing through the bore 16along the exterior of the pipe 24 and then passing through pipe 24 andout the exhaust pipe 28, the gas being heated to a temperature withinthe range of about 220°C. to 240°C., the temperature at which the heater50 maintains the block. In passing over the catalytic material withinthe block, namely the copper tube 24, the copper chips or granules 44,and the copper turnings 46 and 48, nitrogen dioxide is efficientlyconverted to nitric oxide. The inexpensive copper catalyst has beenfound to operate efficiently in the relatively low operating temperaturerange specified over long periods of time without maintenance orrenewal. Moreover, when the gas supplied is dry air of very low humidity(containing nitrogen dioxide, of course) the use of copper granules orchips (of about 20 mesh) appears to avoid the problem of erraticoperation previously experienced with other types of catalysts.

FIG. 4 illustrates the utilization of the converter 10 in achemiluminescent instrument for measuring the concentration of oxides ofnitrogen in ambient air, for example. The sample passes through theconverter 10, nitrogen dioxide being converted to nitric oxide thereby,and is supplied to a chemiluminescent detector head 56, to which ozoneis also supplied at 58. The resultant reaction of ozone and nitric oxideproduces light detected by a photomultiplier tube incorporated withinthe detector 56. The gas stream is drawn through converter 10 anddetector 56 by vacuum pump 60 coupled to the detector via a vacuumregulator 62 and a filter 64. Filter 64, preferably acharcoal-containing cannister, is necessary to prevent corrosion of thepump which would otherwise be caused by the ozone.

In an actual embodiment of the converter, block 12 is seven inches longand one inch in diameter. Bore 16 is 6.75 inches long (including bore20) and 0.281 inches in diameter. Tube 24 is 6.25 inches long with a0.250 inch outer diameter and a 0.030 inch wall thickness. Tube 28 is a0.250 outer diameter tube with a 0.028 wall thickness, while tube 38 isa 0.062 outer diameter tube with a 0.010 wall thickness. The sample gasflow rate is between 400 and 500 cc per minute, and the concentration ofthe NO₂ converted is between 0 and 10 parts per million.

While a preferred form of the invention has been shown and described, itwill be appreciated that changes can be made without departing from theprinciples and spirit of the invention, the scope of which is defined inthe following claims. Although copper chips or granules of the typementioned are the highly preferred catalyst, such chips and granulesoxidize, even at room temperature, and the term copper-containingcatalyst employed in the claims is intended to encompass such oxides andsimilar compounds of copper. Higher order oxides of nitrogen may also beconverted by the invention (all such oxides being referred to as"nitrogen dioxide or the like"). Within the broader aspects of theinvention the gas flow passage may be defined other than by a coppertube within and spaced from the wall of a block bore, although thatarrangement is preferred and provides preheating of the gas along theinterior of the bore. In the preferred form of the invention shown anddescribed (with a flow rate of about 400 to 500 cc per minute and withthe copper chips filling but not tightly pressed into the copper tube)the stated temperature range is appropriate. Below about 220°C. thedesired conversion becomes less efficient, while above about 240°C.conversion of other nitrogen compounds, such as ammonia, may produceinterference. However, within the broader aspects of the invention,other flow rates and particulate packing densities of the catalyst bedmay dictate somewhat different temperatures.

1. A method of converting nitrogen dioxide or the like in a gas tonitric oxide without significant conversion of ammonia, which comprisespassing said gas over a copper-containing catalyst maintained at atemperature
 2. A method in accordance with claim 1, wherein the gas ispassed over a
 3. A method in accordance with claim 1, wherein the flowrate of the gas is between about 400 and about 500 cc per minute.